1. Field of the Invention
The present invention relates to an automatic diaphragm apparatus for a lens which is mounted to a single lens reflex camera. More precisely, it relates to an apparatus for transmitting photometric data related to the diaphragm apparatus to a camera body.
2. Description of the Related Art
The assignee of the present application is also the assignee of U.S. Pat. No. 5,270,759 which discloses a automatic diaphragm apparatus of a lens system. In the automatic diaphragm apparatus, the size of an aperture varies in accordance with a set diaphragm value set by a diaphragm set ring when the set diaphragm value is in the range from the minimum diaphragm value (maximum aperture) to a predetermined intermediate diaphragm value. The size is kept constant regardless of the set diaphragm value when the set diaphragm value is in the range from the intermediate diaphragm value to the maximum diaphragm value (minimum aperture). In this automatic diaphragm apparatus, if the set diaphragm value is changed when the aperture is in the range from the minimum diaphragm value to a predetermined intermediate diaphragm value, a diaphragm effect (soft-focus effect) can be visibly confirmed through a view finder. On the other hand, in the range from the intermediate diaphragm value to the maximum diaphragm value, an automatic diaphragm function is obtained in which the stop-down to the set diaphragm value takes place immediately before the shutter releasing operation. Namely, in the diaphragm range where the field of view of the finder becoming dark should be avoided, the automatic diaphragm function can be obtained without the need for confirmation of the diaphragm effect. The "soft-focus effect" refers to effects caused by spherical aberrations of the lens and are attenuated by the stop-down operation of the diaphragm.
The assignee of the present application is also the assignee of U.S. Pat. No. 5,250,971 which discloses a photometric data transmitting apparatus in the automatic diaphragm apparatus. In the photometric data transmitting apparatus, data indicating the kind of the photometering system used and diaphragm data are transmitted to the camera body. Furthermore, the so-called stop-down metering is carried out in the range from the minimum diaphragm value (maximum aperture) to the intermediate diaphragm value. The so-called full-aperture metering is carried out in the range from the intermediate diaphragm value to the maximum diaphragm value (minimum aperture), respectively. To this end, a diaphragm association member (diaphragm data transmitting member) is provided to transmit the diaphragm data to the camera body, in addition to the diaphragm set ring.
Also, an association mechanism is provided between the diaphragm set ring and the diaphragm association member. the association mechanism retains the diaphragm association member in a predetermined position regardless of the angular position of the diaphragm set ring in the range in which the size of the open aperture changes. The association mechanism also rotates the diaphragm association member together with the diaphragm set ring in the range in which the size of the open aperture does not change.
In the association mechanism, an association spring is provided between the diaphragm set ring and the diaphragm association member to bias the diaphragm set ring and the diaphragm association member in opposite directions. However, the tensile force of the association spring varies depending on the angular position (set diaphragm value) of the diaphragm set ring. Hence the rotational torque of the diaphragm set ring (diaphragm operation ring) is not constant. The click force to click the diaphragm set ring (diaphragm operation ring) at the set diaphragm value must be large enough to resist the maximum biasing force of the association spring. Consequently, a large operational force is necessary resulting in a bad operational perception.